1. Field of the Invention
This invention relates to a silver halide photographic emulsion spectrally sensitized with at least two sensitizing dyes having supersensitizing effects on each other, and more specifically, to a silver halide photographic emulsion having increased spectral sensitivity in the red wavelength region.
2. Description of the Prior Art
One well-known technique of producing photographic materials is a spectrally sensitizing process by which the wavelength region to which a silver halide photographic emulsion is sensitive is broadened to a longer wavelength region by adding a certain kind of cyanine dye. It is generally known that the spectral sensitivity of a silver halide photographic emulsion is affected by the chemical structure of the sensitizing dye and the various characteristics of the emulsion such as the halogen composition of the silver halide, the crystal habit, the crystal system, the silver ion concentration or the hydrogen ion concentration, and also by photographic additives present in the emulsion, such as stabilizers, anti-foggants, coating assistants, precipitating agents, or color couplers.
Generally, only one sensitizing dye is used to sensitize a photographic material to a specific spectral wavelength region. When such sensitizing dyes are used in combination, the sensitivity obtained is often lower than that obtained by using each of the sensitizing dyes individually. However, in some special cases, the spectral sensitivity obtained using a combination of two or more sensitizing dyes makedly increases. This type of sensitization is known as "supersensitization". However, precise selection of sensitizing dyes to be used in combination must be made since a seemingly slight difference in chemical structure can markedly affect the supersensitization of the dyes employed. Accordingly, appropriate combinations of sensitizing dyes having supersensitizing effects are difficult to predict from consideration of their chemical structural formula only.
Generally, the sensitizing effect of a dye on a certain emulsion can be varied by changing the emulsion characteristics. For example, the sensitizing effect can be increased by increasing the silver ion concentration, or by decreasing the hydrogen ion concentration, or by employing both of these techniques. The sensitizing effect can, therefore, be increased by immersing a film coated with the spectrally sensitized emulsion in water or an aqueous solution of ammonia. The above method by which the sensitivity of a sensitized emulsion is changed by increasing the silver ion concentration or decreasing the hydrogen ion concentration or by both is usually called "hypersensitization". Hypersensitized emulsions generally have a short storage life.
When sensitization is applied to a silver halide photographic emulsion, the sensitizing dyes must not have adverse interactions with photographic additives other than the sensitizing dyes, and stable photographic characteristics must be maintained even during the storage of the photographic materials. A further requirement of the sensitizing dyes used is that no "residual coloration" due to the sensitizing dyes in the processed photographic materials must remain. This requirement is especially important when processing the photographic materials rapidly within a short period of time (usually several seconds to up to about 1 minute).
In order to obtain excellent color reproducibility of a color photographic material, the red sensitive layer preferably does not have a high sensitivity at too long a wavelength, for example, at wavelengths longer than 600 nm (the wavelength at which sensitization is maximum), or have a sensitivity at too short a wavelength, for example, at wavelengths shorter than 580 nm (at which sensitization is maximum). According to spectrally sensitizing techniques, it is difficult to increase sensitivity in the wavelength region not exceeding about 630 nm (the wavelength at which sensitization is maximum). Among all, it is particularly difficult to increase the sensitivity in the wavelength region ranging from about 580 to 600 nm, and therefore, to solve this problem is one of the important subjects in the art.